1. Field of the Invention
The present invention relates to a microwave dielectric resonator for operation in the whispering-gallery mode. This resonator is of the planar type or in other words is designed in the form of a flat disk which is either physically distinct from the components with which it cooperates or integrated in a small dielectric plate in which the flat disk is defined by a magnetic wall.
2. Description of the Prior Art
The whispering-gallery (WG) mode was discovered by Lord Rayleigh in the field of acoustics. Thus in a building which has a vaulted gallery architecture a sound as faint as a whisper is transmitted along the vault and is readily propagated over a long distance without loss of energy.
This type of propagation also finds applications in other fields including microwave techniques and the theory has been studied by Vedrenne and Arnaud in an article entitled "Whispering-gallery modes of dielectric resonators" published in IEE Proc. vol. 129, No. 4, pages 183-187, Aug. 1982.
In a cylinder of dielectric material in which an electromagnetic wave is propagated, the solution of the propagation equation makes it possible to define the longitudinal and transverse components of the modes which are capable of propagating. These modes are defined by an azimuthal number (propagation along the axis of the cylinder) and a radial number (propagation along a radius of the cylinder). In the case of modes having a high azimuthal number, the electric field E and magnetic field H which sustain the wave are confined between a so-called caustic surface and the lateral surface of the dielectric cylinder, which accordingly produces radial confinement.
Using the following notations:
a: radius of cylinder,
a.sub.c : radius of caustic surface,
R: radius of a point at which the waveform is considered,
then
in the case of R&lt;a.sub.c : the wave is evanescent,
in the case of a.sub.c &lt;R&lt;a: the wave is oscillating, PA1 in the case of R&gt;a: the wave is evanescent.
Furthermore, it is known to trap these whispering-mode waves by reducing the diameter of the dielectric cylinder on each side of the disk region in which there exists a wave confined by whispering mode. The external radiation is in fact very weak since a whispering-mode wave is confined within a disk having a thickness 2d in the case of a mode having a high azimuthal number.
The invention therefore proposes to construct the resonators for microwave devices, no longer by means of a cylinder of dielectric material having a length of the same order of magnitude as the diameter as in the prior art but by means of a disk of dielectric or metallic material which has a small thickness in comparison with its diameter and operates in the whispering mode, the frequency of the whispering wave being related to the radius of the disk, to the radius of the caustic surface and to the material employed.
By virtue of the fact that the electromagnetic wave is confined and that the external radiation is very weak, a resonator in accordance with the invention operates on any substrate whether of dielectric or metallic material.
Since this whispering-mode resonator is a flat disk, it may be deposited by screen process or the like or alternatively etched in a ceramic plate.